This invention relates to voice and data transmission on a network with one or more nodes not able to consistently communicate with the other network nodes. These nodes, known as intermittent nodes, are sporadically visible to the other network nodes or modems. This invention specifically concerns the management of an intermittent node presence to prevent voice channel interference and/or channel indeterminacy. Prevention of node or modem interference with one another is a prime consideration of the invention.
Poor quality communication channels having time and frequency variable transfer functions (noise levels, impedance levels, frequency response, etc.) have a tendency to intermittently mask one part of the network from other parts of the network. From a given vantage point on the network, this masking has the effect of intermittently adding and removing elements, such as nodes/modems, from the field of view (the apparent network of active and permanent nodes and modems visible at a given time).
Network communication performance is degraded and maybe totally destroyed by the effects of this intermittent masking. Voice channel collisions within a network (e.g., single LAN) results in destruction of information on one or all the contending voice channels when two network segments (e.g., LAN segments) become jointly visible. A system designed to be tolerant of these effects minimizes errors and maximizes voice transmission reliability.
Intermittent masking of nodes in a communication network is prevented from unnecessarily debasing existing voice channel connections by identifying intermittent nodes as each are presented to a portion of the network, and by remembering the intermittent nodes for an extended operational duration (i.e., a duration substantially exceeding any expected voice channel connection duration) as existing permanent nodes. The information regarding the existence of intermittent nodes is communicated throughout the network. The acknowledged existence of the intermittent nodes is included in a database and used as primary information in control of the creation of any newly requested voice channels. Voice channel allocation is coordinated between Sub-LANs that are intermittently isolated during periods where communication is available to minimize contention of voice channels.
In a specific embodiment the method of the invention is included as part of an access control protocol for a local area network (LAN), included in what is commonly called the MAC layer. This protocol is alternately part of the data link layer in the Open Systems Interconnection (OSI) model.
An illustrative embodiment of this invention is shown herein for a combined voice and data communication system, having communication links supporting both voice channels and data. In this embodiment both the active and intermittent nodes are retained in either a central database or a distributed database. If more than one voice signal (i.e., in a time division multiplexed voice communication arrangement) is presented for transmission the database listing provides both active and intermittent nodes as equals for consideration as a signal source to be granted access to a channel (i.e., time slot). Hence the intermittent node, by inclusion in the database, is eliminated as an unknown source of an interfering channel.
According to the invention the database listing is continuously updated. Intermittent nodes/modems are retained in the database listing for a substantial duration. After that time duration expires they are removed. Time slots/channels for any newly occurring voice signals are granted only after a review of activity of the nodes (i.e., both active and intermittent) in the database listing that are actively connected to channels (i.e., time slots). Time slots (i.e. channels) are investigated to determine their use. If any time slots (i.e., channels) are presently available (i.e., not used and not connected to a node listed in the database listing) they are assigned to receive voice signals from any other node listed in the database listing. (EXCEPTION: nodes determined to be intermittent at a specific time are not permitted to connect to each other.) Methods of assignment to minimize node interference during periods of xe2x80x9cdisconnectxe2x80x9d between sub LANs, of an overall LAN system, may be used, such as starting with low number slots for sub a LAN-A and high number slots for a sub LAN-B, or as random channel assignments on all LANs. Optimization schemes, such as odd and even selections or other schemes may be used.
The invention is however not limited to this selection methodology. Other schemes will readily suggest themselves to those skilled in the art. Nodes not listed in the database listing are refused voice channel access until they are registered and recognized as node members of the network. If the database listing indicates that a certain number of nodes are assigned channels; newly active nodes must receive channels only from unassigned channels.
In the particular illustrative embodiment, disclosed herein, a communication system is envisioned in which both voice and data are transmitted. Voice is time division multiplexed into time slots (i.e., channels) and data signals are transmitted as a packet included as part of an over all frame which includes the voice channels. The voice channel section is normally fixed in duration while the data portion is variable in duration. Synchronization of the overall frame and its integral parts (i.e., voice and data) is maintained to prevent interference or clashing of the two parts of the frame with each other (i.e., such as the data portion of the overall frame intruding into the voice channel assigned section).
Synchronization of the frame is achieved through a master reference clock, by which synchronization is maintained throughout all system LANs and LAN segments.
An illustrative example of an application of the invention is in communication systems using power lines as communication links. Control methods, embodying principles of the invention, are utilized in this normally noisy environment. Power lines characteristically have high levels of noise and interference, multiple reflections, time varying transfer functions and stringent EMC requirements. These characteristics tend to mask system modems intermittently.